Mobile communication systems are being standardized to implement efficient and high throughput of downlink (DL) packet data transfer mechanisms. In the context of universal mobile telecommunications system (UMTS), the current working assumption in 3GPP (3rd Generation Partnership Project) is that the access technique for the enhanced UTRAN (E-UTRAN) will be orthogonal frequency division multiplexing (OFDM), which will open for the opportunity to do link adaptation and user multiplexing in the frequency domain.
In order to be able to do this adaptation in the frequency domain, it is crucial that the packet scheduler and link adaptation units in the base station, hereinafter designated as “Node B” in line with UMTS terminology, have knowledge of the instantaneous channel quality. This is obtained through the signalling of channel quality indication (CQI) reports from the different wireless transmit and receive units, hereinafter designated as “user equipment (UE)” in line with UMTS terminology.
A method of how to derive the reported CQI value has been standardized. In the FDD (Frequency Division Duplex) standard, there is a table (as shown in 3GPP TS 25.321, Medium Access Control (MAC) Protocol Specification, 5.4.0 (2003-03)) listing some 30 CQI values roughly corresponding to increasingly higher data rates, and therefore proportional to higher and higher DL signal-to-interference ratios (SIRs). The reported CQI in FDD is derived as follows (per 3GPP TS 25.214, Physical layer procedures (FDD), v5.4.0 (2003-03), section 6A.2): “the UE shall report the highest tabulated CQI value for which a single HS-DSCH sub-frame formatted with the transport block size, number of HS-PDSCH codes and modulation corresponding to the reported or lower CQI value could be received in a 3-slot reference period ending 1 slot before the start of the first slot in which the reported CQI value is transmitted and for which the transport block error probability would not exceed 0.1.” In the TDD (Time Division Duplex) standard, reporting is different; the transport block size is reported if it was transmitted during the last received transmission interval and that transmission would have yielded a block error rate (BLER) of 0.1.
As an example, in the W-CDMA FDD specification, the CQI is an information bit sequence five bits long which is encoded by means of a (20, 5) Reed-Muller code. The resulting 20 bit long coded sequence is sent in the UL on a High-Speed Dedicated Physical Control Channel (HS-DPCCH). Every user has a separate HS-DPCCH with an adjustable CQI reporting cycle (feedback rate). A user can report the CQI on the HS-DPCCH even if the user does not receive data on the HS-DSCH.
As another example, in the W-CDMA TDD specification, the CQI is an information bit sequence ten bits long which is encoded by means of a (32, 10) Reed-Muller code. The resulting 32 bit long coded sequence is sent in the UL as part of the HS-SICH (High-Speed Signaling Channel). With current TDD, a CQI transmission can only take place if the user has previously received data on the HS-DSCH in the frame.
However, there will be measurement errors and potential bias values related to the received CQI values. This problem has been handled by using an outer loop algorithm to compensate for this issue.
An outer loop link adaptation mechanism based on the ACKs/NACKs from past transmissions has been proposed in D. W. Paranchych and M. Yavuz, “A method for outer loop rate control in high data rate wireless networks,” in Proceedings of the IEEE Conference on Vehicular Technology, September 2002, and in M. Nakamura, Y. Awad, and S. Vadgama, “Adaptive control of link adaptation for high speed downlink packet access (HSDPA) in W-CDMA,” in Proceedings of the 5th International Symposium (WPMC'2002). Such a mechanism can be realized by introducing a continuously adjusted CQI offset value, for instance, which is subtracted from all received CQI values in the Node B before an appropriate modulation and coding scheme (MCS) is selected. This can be applied on a per-user level as well as on a per cell level, while the default operation is on a per-user level. Thereby, it is possible to actively control the residual frame error rate after a certain number of retransmission attempts.
Furthermore, the US20060089104 discloses a methodology for improving a high speed downlink shared channel (HS-DSCH) transport format allocation in communication systems (e.g., mobile phone networks) using, e.g., a network element such as a Node B. As CQI reports made by a user terminal, such as a UE, are time stamped in a sense that they correspond to a given reference period, the Node B is able to determine what time instant in the past the given CQI report corresponds to. As the Node B scheduler knows a history of HS-DSCH (high speed downlink shared channel) transmission, it is able to determine how much HS-DSCH power was transmitted during the time corresponding to the received CQI report. Based on this information, it determines the bias required to the CQI reports received at different times to improve an accuracy of the allocated HS-DSCH transport format. Additionally, uncertainty of the UE CQI reports can be partly compensated for by monitoring received ACK/NACK messages for previous transmissions. Hence, an allocated HS-DSCH transport format is either positively or negatively “biased” to adjust an ACK/NACK ratio towards a desired target. Thereby, accuracy of a current “outer loop” algorithm (based on the received ACK/NACK messages) can be improved, which leads to an improved HS-DSCH link adaptation and scheduling performance.
A promising CQI reporting method is to use a tree-based approach, which will rely on using time staggering and partial updates to gradually improve the frequency domain resolution as well as improving the observed measurement error. Thereby, the requirements for accuracy, flexibility, and low-signalling bandwidth can be met. However, a problem using this tree based approach is that the measurement error on the CQI reports will be dependent on the bandwidth as well as the time that the CQI has been measured over. This means that the approach of using a single outer loop algorithm to handle the potential errors made by CQI reporting might prove insufficient.